Mem. S.A.It. Suppl. Vol. 6, 86 Memorie della c SAIt 2005 Supplementi

Peculiar Near-Earth Objects

S. Marchi1 M. Lazzarin1 and S. Magrin1

Dipartimento di Astronomia, Universita` di Padova, Italy e-mail: [email protected], [email protected], [email protected]

Abstract. In this paper we present visible and near-infrared spectroscopy of 7 peculiar NEOs. These objects have been observed within the SINEO (Spectral Investigation of Near- Earth Objects) survey, which have produced more then 100 spectra of NEOs. Among the objects presented here, five of them result to be spectrally peculiar, belonging to the rare V and R classes. The other two have peculiar orbits being comet-like. Their spectroscopy gives a B- and a X-type which is compatible with the scenario of being objects which originated in the outer Solar System.

Key words. minor planet – asteroids – near-Earth objects

1. Introduction bodies and two having comet-like orbits and a primitive composition. Much progress in understanding the origins of near–Earth objects (NEOs) has been accom- plished in the last three decades. NEOs have 2. Igneous bodies in general chaotic, short–living orbits (typical life–times of the order of a few million years), 2.1. V-type suggesting that they have to be continuously In figure 1 the visible spectra of four V-types resupplied from an external reservoir. Two ma- observed NEOs are shown. For each body, the jor sources have been identified (for a review spectrum of the asteroid 4 Vesta has been over- see Morbidelli et al. (2002)). The principal plotted for a direct comparison. one is the Main Belt, where gravitational per- They exhibit an excellent spectral match turbations, mainly by Jupiter and Saturn, cre- with Vesta. We also report the range of varia- ate dynamically resonant regions which pro- tion concerning the previously defined V–type vide escape routes (in this case we refer to asteroids on the basis of 39 V–type objects near–Earth asteroids, NEAs). The second (by found within SMASSII (Bus 1999). The pre- far less important) source is represented by ex- sented NEAs are significantly more similar to tinct comets: a certain number of NEOs may 4 Vesta than the previously known V–type MB represent the final evolutionary state of comets, asteroids. The same is true within the sample of i.e. a devolatilized nucleus. V–type NEAs (a total of 15 bodies, see Binzel In this paper we present the spectroscopy of et al. (2002)). The main differences between some peculiar NEOs: five peculiar igneous 4 Vesta and the other V–types are the slopes in the range 0.4-0.72 µm (from moderately to Send offprint requests to: S. Marchi much redder than Vesta) and the depth of the 1 Marchi: Peculiar NEOs 87 µm band (deeper than Vesta’s: see Florczak et al. (2002); Burbine et al. (2001); Binzel & Xu 2.5 (1993)). We are aware that neither differences 2003 GJ21 in slope do necessarily indicate differences in

2 2003 FU3 composition, as shown by Kelley et al. (2003), nor the similar spectral behaviour between two

2003 FT3 objects necessarily establishes a genetic link. 1.5 However, the similarity between 4 Vesta and Reflectivity

2003 EG the NEOs, whose observations are presented in 1 this paper, stands as a striking feature. For this V-type max V-type min reason, in the following we shall compare our (4) Vesta

0.5 data not only to V–types in general, but also of- 0.4 0.5 0.6 0.7 0.8 0.9 1 Wavelength (µm) ten to Vesta. For two NEAs in our sample, namely 2003 FT3 and 2003 EG, we performed also NIR ob- servations; unfortunately these spectra are too

2003 FT3 noisy for a quantitative analysis of bands posi- 1.4 (4) Vesta (2579) Spartacus tion and depth, which would be necessary for a detailed compositional analysis. 1.2 The composite visible-NIR spectrum of 1 2003 EG (Fig. 1, lower panel) resembles that

Reflectivity of 4 Vesta throughout all the observed range, 0.8 and in particular the shape of the 1 µm band is similar. However, the depth of the 1 µm band 0.6 is not the same, being deeper on Vesta than on

0.5 1 1.5 2 2.5 2003 EG, perhaps indicating a different con- µ Wavelength ( m) tent of pyroxene and/or olivine. Unfortunately, the 2 µm band is out of the measured range, 1.8 thus preventing any detailed compositional analysis. Anyway, the high reflectivity level of 1.6 2003 EG 2003 EG at 1.6 µm suggests a lower content of (4) Vesta 1.4 pyroxene with respect to Vesta. The overall NIR spectrum of 2003 FT3 (Fig. 1.2 1, middle panel) is consistent with the V–type, Reflectivity

1 but the right–hand limb of the 1µm band is very peculiar and much different from other 0.8 V-types (and from Vesta as well). Moreover the 2 µm band is less deep than that of Vesta. 0.6 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Wavelength (µm) Similar features, for the first time detected among NEOs, have been observed at least Fig. 1. Upper panel: Comparison between the ob- in another MB V–type object, namely 2579 served NEOs (namely, 2003 EG, 2003 FT3, 2003 FU3, and 2003 GJ21) and 4 Vesta in the visible re- Spartacus (Burbine et al. 2001). We stress that gion. The spectra are shifted vertically for clarity. the spectra of Vesta, Spartacus and 2003 FT3 For 2003 EG the maximum and minimum spread- are rather different one from each other. ing among V-types (Bus 1999) is also shown. However, the broad 1 µm bands of Spartacus Middle panel: Comparison between 2003 FT3 and and 2003 FT3 may suggest a compositional 4 Vesta. The Main Belt asteroid 2579 Spartacus is similarity (note also that no similar features also overplotted (Burbine et al. 1997). Lower panel: have been detected among HED meteorites). Comparison between 2003 EG and 4 Vesta. The The peculiar spectral features can be seen smoothed specrtum of 2003 EG is also shown, for (here as for 2579 Spartacus, see Burbine a easier comparison. 88 Marchi: Peculiar NEOs

havior of igneous, V-like, objects. However its visible spectrum also presents many common aspects with another group of igneous bodies, 1,4 Vesta MS-CMP-042-B (Laser irrad. oliv.) 0.6 Vesta + 0.4 Oliv. those belonging to the R-class. Tholen et al. 2003 FT3 1,2 (1989) introduced the distinction between V- and R-class on the basis of the spectra of two

1,0 peculiar MBAs, 4 Vesta and 349 Dembowska,

Reflectivity respectively (for further details, see Gaffey et

0,8 al. (1993)). Recently Bus (1999), still kept this distinction, although he provided new

0,6 spectra in both classes increasing the spreading between each class. In particular he found 0,5 1,0 1,5 2,0 2,5 Wavelength (µm) 3 more objects similar to 349 Dembowska, and hence classified as R-types. However, a detailed inspection of all the visible spectra Fig. 2. Linear combination of Vesta’s spectrum with laser irradiated olivine. The percentages which give of V- and R-types show that this division is the best fit are 60% Vesta and 40% olivine (see text somewhat weaker than that introduced by for further details). Tholen. According to this classification, 2001 XR31 would be at the border between V- and R-class (see Fig. 3, upper panel). However, we et al. (2001)) as diagnostic of an excess underline also that the spectrum of 2001 XR31 of olivine and/or plagioclase, compared to surprisingly overplots exactly the R-types in typical V–types. Indeed, olivine has a broad the range 0.4-0.85 µm. Moreover, we point profile at the right side of the 1 µm band and out another similarity between the R-types has no 2 µm band (e.g. see Hiroi & Sasaki and our NEO, namely on the basis of their (2001)) while plagioclase has a wide band that behavior within the range 0.86-0.95 µm. In extends from 1 to 1.5 µm, and has no 2 µm fact, the reflectivity of all the R-type objects band. In order to investigate such hypotesis, in this interval is noticeably constant (see Fig. we computed a linear combination (e.g. see 3, upper panel) while for the V-types it is Hiroi et al. (1993)) of Vesta spectrum with generally slightly decreasing. In other words, the spectrum of olivine (taken from RELAB, the bottom of the 1µm band is almost flat. http://www.planetary.brown.edu/relab/). The Now, it is difficult to understand the origin of result is shown in fig. 2. Although the linear this behavior, but it is believed to be significant combination could represent a oversimpli- for compositional differences. We point out fication of the actual 2003 FT3 surface (it that it could be due to mixture of compounds holds only if the diverse species have separate which have the peak of the 1 µm band at locations on the surface), an excellent match slightly different wavelengths, for example by has been achieved. We recall that a spot of a mixture of olivine and pyroxene or a mixture olivine has been identified (Gaffey (1997); of pyroxenes with different contents of Fe (e.g. Thomas et al. (1997); Cochran & Vilas see Gaffey (1997)). To better show the dif- (1998)) on the surface of Vesta: 2003 FT3 ferences between V-types and R-types, in Fig. might originate from a surface layer of Vesta, 3 we also report the object 3155 Lee which, close to the spot. among the V-types, is the one which closer For a more detailed analysis see also Marchi resembles 2001 XR31. Fig. 3 clearly shows et al. (2005). that although the overall behavior of these two objects is quite similar, there are differences in slope and the shape of the 1 µm band, 2.2. R-type reasons for which the spectrum of 2001 XR31 In Fig. 3 the visible spectrum of 2001 XR31 is best fitted by the R-types, instead of V-types. is shown. Its spectrum presents the overall be- Marchi: Peculiar NEOs 89

Other possible reasons able to increase the reflectivity could be compositional differences (349) Dembowska or space weathering. If 2001 XR31 is a (1904) Massevitch 1.2 (2371) Dimitrov (5111) Jacliff fragment of 349 Dembowska, their differences could be affected both by a different degree of space weathering and/or compositional 1 differences. Finally, the steep right-hand limb

Reflectivity of the 2µm band of 2001 XR31 would indicate 2001 XR31 (3155) Lee a greater concentration of pyroxene on 2001 0.8 Mean V-type XR31 with respect to 349 Dembowska.

0.6 0.4 0.5 0.6 0.7 0.8 0.9 1 Concerning the intriguing possibility that µ Wavelength ( m) 2001 XR31 (having an estimated diameter of about 1.3 km) is related to R-type instead of V- type, notice that the only possible known par- ent body is 349 Dembowska. This is because

2 2001 XR31 it has a diameter of about 140 km, while the 349 Dembowska (52color) 1904 Massevitch (SMASSIR) other main belt R-types have diameter in the (4) Vesta (3155) Lee range 6-12 km, unless they are remnants of 1.6 a larger R-type body which suffered a colli- sion, but there is no evidence for that at the Reflectivity 1.2 moment. Nevertheless other sources cannot be excluded. The surface of 349 Dembowska is

0.8 still poorly known. The only information about its shape comes from the analysis of the light

0.4 0.6 0.8 1 1.2 1.4 1.6 curves (e.g. see Zappala` et al. (1979)). As µ Wavelength ( m) pointed out by Abell & Gaffey (2000), the light curves and spectral variations seem to in- Fig. 3. Upper panel: Comparison of 2001 XR31 dicate the presence of an albedo spot near the with R-type MBAs (taken from Bus 1999) in the equator. Possibly it could be a trace of a crater- visible region. The mean V-type and 3155 Lee are ing event, as also noted by Abell and Gaffey. If also shown (both from Bus (1999)). Lower panel: so, the differences observed in the NEOs spec- Comparison of 2001 XR31 with R-type asteroids. tra with respect to the R-types can results from On the same plot, 4 Vesta and 3155 Lee are also NEOs being fragments of deepen layers which shown. The NIR of 1904 Massevitch and 3155 Lee should have different composition considering are from SMASSIR (Burbine et al. 1997), while the spot albedo variation. the NIR of 349 Dembowska is from 52-Color sur- vey (Bell et al. 1988). However, it has a proper semi-major axis of 2.925 AU which falls inside the 5:2 and 7:3 mean motion resonances. These resonances are probably not too efficient to produce NEOs, be- However, the NIR (see Fig. 3, lower panel) cause they tend to push objects into high eccen- of 2001 XR31 does not resemble those of the tricity orbits and eventually, when they reach only two available R-types. Anyway, the 2001 a critical eccentricity for becoming Jupiter XR31 high values of reflectivity in the range crosser, the objects are subsequently removed 1.3-1.5 µm with respect to 349 Dembowska is from the inner solar system by close encoun- likely the same kind of discrepancy shown by ters with Jupiter (see Moons & Morbidelli the V-types and Vesta. For the latter a possible (1995); Gladman et al. (1997); Morbidelli & explanation has been found in grain size Gladman (1998)). However this possibility effects (Burbine et al. 2001): smaller particle cannot be ruled out, because for eccentricity sizes produce higher values of reflectivity. lower than the critical value, some objects can 90 Marchi: Peculiar NEOs have close encounter with Mars and the Earth (see Moons & Morbidelli (1995); Morbidelli 2,0 & Gladman (1998)), and eventually, they can be trapped in near-Earth space, as maybe indi- 1,8 (65996) 1998 MX5 rectly prove by the presence of many C-types 1,6 among NEOs. Moreover, notice that fragments 1,4 released from Dembowska could easly enter the 7:3 resonance, for which a velocity of about 1,2 Reflectivity 0.1 km/s is sufficient; while higher velocity 1,0

(∼0.5 km/s) are needed to reach the 5:2 reso- 0,8 nance. Also taking into account possible semi- 2002 UN major axis mobility, i.e. the Yarkovsky effect 0,6 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 (Farinella & Vokrouhlicky 1999), it seems un- Wavelength (µm) likely that the 5:2 has played a role in deliver- ing fragments from 349 Dembowska. For further details see Marchi et al. (2004). Fig. 4. Visible and NIR spectroscopy of two NEOs having comet-like orbit.

3. Primitive bodies In this section we present the spectroscopy of two primitive NEOs (i.e. 1998 MX5 and 2002 UN), having comet-like orbits. They, in sified as V–type. Moreover, considering fact, have Tisserand invariants with Jupiter their good spectral match with 4 Vesta (TJ) of 2.95 and 2.81, respectively. We re- they can be defined as Vesta analogues. mind that objects with TJ < 3 are usually They are among the objects which more considered comet-like asteroids. This dynam- closely resemble Vesta itself observed so ical condition has been strengthened by our far; in particular 2003 EG shows a striking observations: both objects are indeed B- X- spectral similarity to Vesta both in the types. The reddish and featureless spectra of visible and in the NIR. 1998 MX5 and 2002 UN are typical of prim- – Due to the spectral match with Vesta and itive objects, cometary nuclei and most as- considering their dimensions, the presented teroids in cometary orbits studied in the 0.5- objects could be fragments from the sur- 2.4 µm range. Unfortunately the very few face layer of 4 Vesta. 2003 FT3 shows cometary nuclei spectroscopically observed in the presence of an excess concentration of this wavelength range did not show any kind of olivine. Its properties might be consistent peculiar absorption feature (e.g. Lazzarin et al. with an origin close to the known olivine (1996), De Sanctis et al. (2000), Licandro et spot on the surface of Vesta. al. (2002), Licandro et al. (2003)). Licandro – 2001 XR31 resembles V-type bodies, but et al. (in preparation), also observed 17 aster- we point out its possible link with the R- oids in cometary orbits, and found that 15 of class. If this would be confirmed by other them present featureless, slightly red spectra, observations, it will be the first R-type dis- and failed to find any feature that could dis- covered among NEO so far. Considering tinguish between them and main belt primitive the possibility that 2001 XR31 is R-type, asteroids. we point out that it could have been in- For further details see Lazzarin et al. (2005). jected in near-Earth space by the 7:3 mean motion resonance with Jupiter. 4. Conclusions – Two other NEOs having comet-like orbits, have been revealed to be primitive, con- – Four NEOs (2003 EG, 2003 FT3, firming their possible origin in the outer 2003 FU3 and 2003 GJ21) have been clas- Solar System. Marchi: Peculiar NEOs 91

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